Smoke rises from the front line of a clash south of Kirkuk, Iraq, on Saturday, March 14.

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A group of Kurdish Peshmerga troops take a break from fighting ISIS militants south of Kirkuk on March 14.

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Iraqi security forces and allied Shiite militiamen gather in Tikrit on Friday, March 13. Ousting ISIS from Tikrit is important for the United States-led coalition trying to thwart the extremist group's quest to grow its caliphate. ISIS wants to create an Islamic state across Sunni areas of Iraq and Syria.

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A motorcyclist passes destroyed buildings in the Syrian town of Kobani on Thursday, March 12, after it has been freed from ISIS militants.

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Smoke billows after a mortar shell hit an Anbar governorate building in Ramadi, Iraq, on Wednesday, March 11. ISIS has launched a coordinated attack on government-held areas of the western Iraqi city. Ramadi has been the focus of a fierce ISIS assault since Iraqi forces made gains against the group in Tikrit.

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The parents of 19-year-old Mohammed Musallam react at the family's home in the East Jerusalem Jewish settlement of Neve Yaakov on Tuesday, March 10. ISIS released a video purportedly showing a young boy executing Musallam, an Israeli citizen of Palestinian descent who ISIS claimed infiltrated the group in Syria to spy for the Jewish state. Musallam's family told CNN that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.

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Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.

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Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants recently abducted at least 220 Assyrians in Syria.

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Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on Wednesday, February 4. Al-Kasasbeh's son, Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was recently released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.

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A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.

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Smoke billows in Kirkuk as Kurdish Peshmerga fighters take position against ISIS militants on January 30.

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Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.

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Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.

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Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.

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ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.

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An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.

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Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.

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Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.

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A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.

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Iraqi special forces search a house in Jurf al-Sakhar, Iraq, on Thursday, October 30, after retaking the area from ISIS.

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ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.

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A U.S. Air Force plane flies above Kobani on Saturday, October 18.

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Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on October 18.

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Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.

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Turkish police officers secure a basketball stadium in Suruc on October 14. Some Syrian Kurds were held there after crossing from Syria into Turkey. Tens of thousands of people fled Kobani to escape ISIS.

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Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.

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Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.

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In this photo released by the U.S. Air Force on Saturday, October 4, a U.S. Navy jet is refueled in Iraqi airspace after conducting an airstrike against ISIS militants.

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A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.

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Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.

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Tomahawk missiles, intended for ISIS targets in Syria, fly above the Persian Gulf after being fired by the USS Philippine Sea in this image released by the U.S. Navy on Tuesday, September 23.

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Turkish Kurds clash with Turkish security forces during a protest near Suruc on Monday, September 22. According to Time magazine, the protests were over Turkey's temporary decision to close the border with Syria.

Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.

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Iraqi volunteer fighters celebrate breaking the Amerli siege on Monday, September 1. ISIS militants had surrounded Amerli, 70 miles north of Baquba, Iraq, since mid-June.

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Kurdish Peshmergas fight to regain control of the town of Celavle, in Iraq's Diyala province, on August 24.

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Peshmerga fighters stand guard at Mosul Dam in northern Iraq on Thursday, August 21. With the help of U.S. military airstrikes, Kurdish and Iraqi forces retook the dam from ISIS militants on August 18. A breach of the dam would have been catastrophic for millions of Iraqis who live downstream from it.

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Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.

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Peshmerga fighters inspect the remains of a car that reportedly belonged to ISIS militants and was targeted by a U.S. airstrike in the village of Baqufa, north of Mosul, on August 18.

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Kurdish Peshmerga fighters fire at ISIS in Khazair, Iraq, on Thursday, August 14.

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Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

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Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.

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A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.

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A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.

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New army recruits gather in Najaf, Iraq, on Wednesday, June 18, following a call for Iraqis to take up arms against Islamic militant fighters.

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Kurdish Peshmerga forces, along with Iraqi special forces, deploy their troops and armored vehicles outside of Kirkuk, Iraq, on June 12.

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Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.

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Civilians from Mosul escape to a refugee camp near Irbil, Iraq, on June 10.

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Story highlights

Many cases in recent months involve European jihadists returning from Syria and Iraq

"The threat of attacks has never been greater," says a European counter-terrorism official

Last month, a gunman opened fire at a Jewish museum in the Belgian capital, Brussels

Authorities are investigating whether ISIS had a role

The city of Cannes on the French Riviera is best known as a hang-out of the rich, and for its film festival, not as a terrorist hide-out. But police recently swooped on an apartment on its outskirts -- and discovered soda cans converted into crude bombs. The devices contained nearly one kilogram of the high explosive TATP -- a substance used to make detonators in multiple al Qaeda bomb plots against the West in the years after the September 11, 2001 attacks on the United States.

The apartment was the home of a 23-year old man -- named by prosecutors only as Ibrahim B -- who had swapped online messages that talked of "punishing France." He had spent 18 months in Syria fighting with the al Qaeda affiliated Jabhat al Nusra before being arrested in February after returning to France.

The case of Ibrahim B is one of dozens in recent months involving European jihadists returning from Syria and Iraq, after joining groups like al-Nusra and the Islamic State in Iraq and Syria (ISIS) -- gaining combat experience, training and an extremist mindset.

A European counter-terrorism official told CNN up to 300 veterans of the Syrian Jihad have already come back to Europe.

"The threat of attacks has never been greater -- not at the time of 9/11, not after the war in Iraq -- never," the European counter-terrorism official told CNN. He envisaged a flood of small-scale but effective and chilling attacks.

"We must do that not only in Syria, but in Iraq, Somalia, Nigeria and Mali, because these problems will come back and hit us at home if we do not," he said.

An early indicator of the potential threat came last month when a gunman opened fire at a Jewish museum in the Belgian capital, Brussels, killing four people. Authorities are investigating whether ISIS had a role in what was the first terror attack on Western soil linked to Syria.

The man accused of the murders is Mehdi Nemmouche, a French-Algerian extremist who travelled to Syria after being radicalized in a French jail, according to prosecutors. When French police arrested him a week after the attack they found a Kalashnikov wrapped in a flag with ISIS insignia in his possession.

Just hours after his arrest was made public a French ISIS fighter based in Syria tweeted that Nemmouche had fought with ISIS under the name Abu Omar al Firansi. The posting was then deleted, according to a senior Belgian counter-terrorism official.

It was the "strongest indication we have so far that Nemmouche was part of the ISIS but by no means proves it," the official told CNN. But he added: "We can't rule out that he was thinking of launching an attack even before leaving for Syria." ISIS has not claimed responsibility for the attack and Nemmouche has yet to enter a plea.

Aggravating the threat, according to a European-counter-terrorism official, is woefully inadequate intelligence-sharing in Europe. Nemmouche, a French citizen, was on a European watch-list. But he took precautions to try to mask his time in Syria -- flying into Frankfurt from Bangkok in March after spending several weeks in south-east Asia.

German authorities informed French police when he arrived at Frankfurt airport. But Nemmouche traveled to Belgium, where authorities were unaware of his time in Syria. European Union treaties allow individuals to travel across much of the continent without showing their passport.

The official said a central European database was needed to track extremists leaving to and returning from Syria. He said European governments were beginning to understand the need for better collaboration.

There was a sign of that last weekend with the arrest of a French citizen, Tewffiq Bouallag, who arrived in Berlin from Istanbul last weekend. The day before his arrival, France issued an international arrest warrant for Bouallag (there was no such warrant for Nemmouche.) Bouallag had joined ISIS in Syria, according to postings on social media sites, and uploaded photos of himself and other ISIS fighters, encouraged others to follow his example and even indicated he was preparing to return to Europe.

But scouring social media and jihadist forums is labor-intensive and demands language skills. And the most dangerous militants won't be so transparent about their intentions.

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European officials believe about 2,000 EU citizens have answered the call to jihad, including the wives of fighters. Perhaps 500 to 1,000 of them have joined ISIS, officials say.

France, Germany and the UK account for the largest numbers, all with hundreds of citizens fighting in Syria.

But in per capita terms more have travelled from Belgium than any other EU country. A Belgian counter-terrorism official says they are aware of 150 Belgian fighters in Syria -- most of them with ISIS. Up to 15 more were leaving every month. About 35 are thought to have been killed, and 60-70 had returned home. Round-the-clock surveillance is impossible for all but a very small number because of the prohibitive expense.

Belgium has sizeable North African and Turkish Muslim populations and active radical groups such as Sharia4Belgium, some of whose supporters traveled to Syria back in 2012, laying the groundwork for more to follow and communicating with friends via social media to encourage them to make the journey.

One figure of particular concern is Azzedine Kbir Bounekoub -- a Belgian in his 20s who is thought to be in Raqqa in Syria, ISIS' main hub, and who has called for further attacks in Belgium.

"May Allah raise up more young people who take the example of those who committed the attack in the Jewish museum ... their blood, wealth and honor is halal for us," he wrote on his Facebook page on June 3.

The official told CNN that authorities were often playing "catch-up" -- sometimes finding out about suspicious travel months after someone had packed their bags. Inevitably, he said, some escaped detection altogether. The task was made more difficult by the fact that a lot of Belgians go on vacation to Turkey.

European officials say there needs to be closer co-operation with Turkey, which should be given a list of people suspected of intending travel to Syria. Turkish authorities say such individuals should be prevented from leaving their countries of origin, but travel bans are often difficult to impose through the courts. Nor is travel to Syria illegal in itself; authorities have to show membership of designated terror groups or the intention to commit an act of terror in order to bring charges.

It's not solely a European problem either. A group of Canadians is known to be fighting for ISIS. One of them, Salman Ashrafi from Calgary, was eulogized after carrying out a suicide attack that left at least 19 people dead. And a young U.S. citizen became the first American to carry out a suicide bombing in Syria last month.

'Worse than al Qaeda'

As yet neither ISIS nor al-Nusra has shown any intention to export jihad to Europe, but militants returning home may need no encouragement, and may have bomb-making skills. Both groups carry out suicide bomb attacks on an almost daily basis.

"The worry is that Europeans in the group may out of their own initiative return home to launch attacks. The question becomes to what extent will the ISIS leadership try to control this?" said a European counter-terrorism official.

The extremist discourse in Europe is now dominated by ISIS, which "is winning the battle for hearts and minds much more than al Qaeda," thanks to its lightning offensive across Iraq, says the official. It's a replay of the jihadist fervor generated by al Qaeda in Iraq when it captured the city of Falluja in 2004.

"There's huge excitement on online jihadist forums," said the official. "It's a further catalyst for radicalization and could lead to a surge in travel flows."

Some militants return to Europe traumatized and frustrated by the brutality and infighting in Syria and appear to want no further part in jihad, according to counter-terrorism sources. But many would-be jihadists think nothing of brutal videos showing Shia getting their throats slit. And rather than being angered by specific events, such as the U.S. invasion of Iraq, they are motivated by an ingrained loathing of the West.

ISIS is beginning to tap into this vein. This week it issued a five-minute video in German entitled "Haya Alal Jihad" (Let's Go For Jihad) on its Twitter account. According to the SITE Intelligence Group, which monitors jihadist communications, a "voice that sounds like German jihadist Denis Cuspert (Abu Talha al-Almani) calls on Muslims to join jihad and seek martyrdom."

"Brothers, it's time to rise, set forth for the battle if you are truthful, either achieve victory or the shahadah [martyrdom]," the narrator says.